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GALEX Data Description

  • What are the file naming conventions?
  • What are the GALEX pipeline file types?
  • What is the summary description for file naming and types (mission planning)?
  • What is the data content and format?
  • What are the data column names?
  • What is the meaning of the artifact flags in the images?

    Q: What are the file naming conventions?
    A: The automated outputs of the GALEX data pipeline will provide the primary data set for the archive Database Tables. Supplementary data will also be obtained from science operations planning files, calibration data, corollary data sets and tables and/or files generated ex post facto for archival purposes. This section describes data sets which will be ingested or referenced by the archive.

    GALEX file naming conventions

    GALEX file names contain a prefix, a suffix and an extension (e.g. AISCHV2_153_29114_0002_sv11-xd-mcat.fits,

    or

    MISDR1_29079_0383_002-xd-int.fits).

    The prefix contains alphanumeric characters of unspecified length, but contains no hyphens or periods (e.g. AISCHV2_153_29114_0002_sv11 and "MISDR1_29079_0383_002" above). The first 3 characters of the prefix generally map to the survey. Filenames for all-sky survey

    (AIS) data will have _sv## at the end of the prefix, where the ## gives the number of the sub-visit.

    The filename suffix contains delimited file type descriptor strings (e.g. "-xd-" in both cases above). The filename suffix may begin with a band (FUV/NUV) or mode (direct/imaging/opaque) substring:

    • -nd- NUV direct (imaging)
    • -fd- FUV direct (imaging)
    • -xd- Band-merged direct (imaging)
    • -ng- NUV grism
    • -fg- FUV grism
    • -xg- Band-merged grism
    • -no- NUV opaque
    • -fo- FUV opaque

    Note that the file name is preceded by a period. Unless otherwise indicated, files transmitted to MAST are FITS binary files adhering to contemporary usage standards defined in http://archive.stsci.edu/fits/fits_standard/. The responsibility for verification of mission files transmitted in this format rests with the GALEX SOC. The file extension names are listed as follows:

    .fits
    Any FITS formatted file.

    .txt
    An ascii text file.

    .

    Filename extensions - pipeline files

    Filename extensions are preceded by a period. Unless otherwise indicated, files transmitted to MAST are FITS binary files adhering to contemporary usage standards defined in http://archive.stsci.edu/fits/fits_standard/. The responsibility for verification of mission files transmitted in this format rests with the GALEX SOC. The file extension names are listed as follows:

    .fits
    Any FITS formatted file.

    .txt
    An ascii text file.


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    Q: What are the GALEX file types?
    A:
    Table 1:   Pipeline Products: science data files
    Filename suffix File type Description Files available in Visit, or Coadd directory (or Both)?
    Low-level pipeline products   Photon & housekeeping files  
    -asp.rec Binary record Satellite attitude solution. Raw S/C pointing data. V
    -asprta.rec Binary record Refined spacecraft attitude solution (using bright stars in the FOV). V
    -rtastar.fits FITS binary table Star catalog for attitude refinement: info. on stars used for S/C pointing solution vs. time. B
    -scst.fits FITS binary table Spacecraft state file: S/C status values. V
           
    Images   Maps & associated files  
    -[n or f][d or g]-cnt.fits FITS image Count map (J2000) in counts (photons) per pixel, corrected for image position and dither. B
    -[n or f][d or g]-exp.fits FITS image Exposure map (J2000). An image of time spent observing within the GALEX FOV (w/o response correction). B
    -[n or f][d or g]-intbgsub.fits FITS image Background-subtracted intensity map (J2000). Note: "-intbgsub" equals "-int" minus "-skybg". B
    -[n or f][d or g]-int.fits FITS image Intensity map (J2000). GALEX FOV image in counts per sec. per pixel, corrected for relative response. B
    -[n or f][d or g]-movie.fits FITS image cube Time-slice count maps (J2000). Multiple images of the raw detector within selected segments of time (not corrected for S/C motion). V
    -[n or f][d or g]-rr.fits FITS image Low resolution relative response (J2000). A low resolution version of "rrhr". (Provided only for rrhr and beyond.) B
    -[n or f][d or g]-skybg.fits FITS image sky background image (J2000) B
    -[n or f][d or g]-wt.fits FITS image weight/threshold image (J2000). FOV image used by Sextractor for thresholding sources. Pixels with values > 1 have a flux above the detection threshold. B
           
    Grism-specific files   Spectral 1-D arrays or 2-D images  
    -[n or f]g-gsax.fits FITS binary table Spectral extraction parameter information for each source B
    -[n or f]g-pri.fits FITS binary table Image strips for each source from a single visit V
    -[n or f]g-prc.fits FITS binary table Image strips (stacked) for each source from multiple visits C
    -[n or f]g-prm.fits FITS binary table Image strips (median) for each source from multiple visits C
    -[n or f]g-gsp.fits FITS binary table Spectral data for each extracted source vs. wavelength B
    -xg-gsp.fits FITS binary table Spectral data for each extracted source vs.wavelength (both bands) B
           
    Quality Assurance (QA)   QA file description  
    -x[d or g]-int_2color*.jpg FITS binary table JPEG images of the field. B
    *.ps postscript text Various postcript plots B
    -[n or f]d-dphcent.txt ASCII text Aspect refinement QA data V
    -[n or f]d-gauss*.txt ASCII text Gaussian filter used by Sextractor B
    -[n or f]d-sexcols.txt ASCII text Sextractor catalog columns B
    -[n or f]d-sexparams.txt ASCII text parameter file used by Sextractor B
    -[n or f]d-sexnnw.txt ASCII text neural network parameter file used by Sextractor B
    *stats.txt ASCII text Various QA statistics. B
    -[n or f]d-cat.ds9reg ASCII text ds9 region file: ellipses of sources listed in the -[n or f] d-c at.fits file. [Note: a ds9 region file is a file of ellipse parameters used b y t he image viewing program "DS9". These parameters include the RA, Dec, major a nd minor axes and position angle of the major axis.] B
    -fd-ncat.ds9reg ASCII text ds9 region file: ellipses of sources listed in the -fd-ncat. fit s file. B
    -nd-fcat.ds9reg ASCII text ds9 region file: ellipses of sources listed in the -nd-fcat. fit s file. B
    _getmask-[n or f]d-cat.ds9reg ASCII text ds9 region file: Objects ignored in estimating background flux. B
    -rtastar.ds9reg ASCII text ds9 region file: ellipses of bright stars from -rtastar.fit s file used in the attitude solution (aspect correction). V
           
    Catalogs   NUV, FUV, and merged catalogs  
    -[n or f]d-cat.fits FITS binary table Sextractor catalogs for images. Table of objects extracted by GALEX reduction pipeline. The table contains positions, flux, magnitude, and major/minor axes. B
    -[n or f]d-[f or n]cat.fits FITS binary table NUV (FUV) extractions using FUV (NUV) positions; the latter are taken from the FUV (NUV) source catalog "fd-cat.fits" ("nd-cat.fits"). B
    -xd-mcat.fits FITS binary table Merged source catalog. Band-merged table of extracted objects. Contains all objects contained in "[n or f]d-cat.fits" matched to the best candidate. The table contains positions, flux, magnitude, and major/minor axes. By definition, it contains all data from the two single-band catalogs. C
           
    Flags   Artifact flag information  
    -[n or f][d or g]-flags.fits FITS image Artifact Flag image (J2000). Identifies predetermined regions which may introduce systematic errors in data extraction. These artifact regions are caused mainly by reflections of nearby bright stars. B
    -[n or f]d-flagstar.fits FITS binary table Star catalog for flagging. List of known bright stars used to determine artifact regions. V


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    Q: What is the summary description for file naming and types (mission planning)?
    A: Mission planning files are created by the GALEX science operations center in the process of scheduling science observations. All mission planning files are text or FITS files. FITS files have the same names as text files with a .fits extension added. Extension names and types for standard planning files are listed below. Files that will be ingested into the MAST Database are tagged with a * symbol. Certain classes of files (e.g. *.rec files) may be retired soon and may not exist in future internal or public release; again, those files ingested directly into the Database are tagged with *. 

    Note addition of * and .fits in the extension names below. The pend_obs file is an addition.

    * acs.fits
    Attitude control command file (for each eclipse).
    * ins.fits
    Instrument settings file (for each eclipse).
    etf.fits
    Eclipse time file.
    obs.fits
    Observe file
    * pdb.fits
    Plan data base. Contains list of plans.
    * tdb.fits
    Target data base. Contains list of targets.
    * pend_obs.fits
    Observations pending. Contains observation list.

    For further information on the pipeline data description, goto CalTech's Pipeline and advanced data description
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    Q: What is the data content and format?
    A:
    Photon and housekeeping level

    These files contain binary FITS tables which will not be stored in the Data Base but along with calibration files are not ordinarily used or accessible by users through it. Database metadata and Table entries may be extracted from FITS headers if necessary.

    -scst.fits
    Aspect solution and state record file. Will be generated from hkfile and hksc and modified by RTA.

    -raw6
    Raw time-tagged photon data Single visit, single tile raw6 photon records. Time-tagged binary FITS table of photon records. May be broken down into segments
    	T       time in seconds
    	Byte1   8-bits of photon X, Y, Q, XA data
    	Byte2   8-bits of photon X, Y, Q, XA data
    	Byte3   8-bits of photon X, Y, Q, XA data
    	Byte4   8-bits of photon X, Y, Q, XA data
    	Byte5   8-bits of photon X, Y, Q, XA data
    	Byte6   8-bit time offset from photon header time
    	flags
    

    -x
    Extended photon record. Processed, time-tagged photon data

    Primary image null. Binary FITS table (extension 1) with 11 fields per row.

     T	Time
     x, y	detector position
     X,Y,Z	J2000 position of photon in X,Y,Z coordinates (unit sphere)
     val	photon XA value
     q	photon pulse height
     flags	photon flag
     Thetax, Thetay	Position of photon in focal plane
    

    Target level - image files

    These files contain binary FITS images which are not to be stored in the database, but which the archive user will be able to download upon command. Database metadata and table entries will be extracted from FITS headers.

    Postage stamp binaries may be generated and stored in database from some of the images described below.

    -(n or f)d-cnt
    Total count map (J2000) cnts pixel

    FITS 2-D image. Dimensions are RA--TAN, DEC-TAN. Header uses FITS WCS standard. Units are in counts.

    -(n-f)d-rr
    is sensitivity times exposure time (i.e. seconds/pixel)

    FITS 2-D image. Dimensions are RA--TAN, DEC-TAN. Header uses FITS WCS standard. Units are in cm s Å .

    -(n or f)d-int
    is Photons (counts) per pixel per second corrected for the relative response. Note that "-int.fits" is equal to "-cnt.fits" divided by "-rrhr.fits".

    FITS 2-D image. Dimensions are RA--TAN, DEC-TAN. Header uses FITS WCS standard. Units are in cnts pixel cm s Å

    -(n or f)d-exp
    is an image of the time (in seconds) spent observing within the GALEX field of view (no response correction). When computing magnitudes (consistent with our -mcat catalog) use the formula: MAG = -2.5 * log_10( FLUX ) + ZPMAG
    where FLUX is in counts per second (from the -int image), and where ZPMAG = 20.08 (NUV) and 18.82 (FUV).

    FITS 2-D image. Dimensions are RA--TAN, DEC-TAN. Header uses FITS WCS standard. Units are in cnts/ pixel cm s Å

    -(n or f)d-wt
    Weight map/Mask for source extraction

    FITS 2-D image. Dimensions are RA--TAN, DEC-TAN. Header uses FITS WCS standard. Units are in cnts/ pixel cm s Å

    -(n or f)d-cat
    Extracted source catalog from single band

    Primary image null. Binary FITS table (extension 1) with 49 fields per row. Fields follow standard (or modified) Sextractor outputs: Isophotal (standard and corrected), aperture, Kron, ``Best'' magnitudes and errors, Background, detection threshold, centroid position (image, world coordinates), peak position, shape parameters (semi-major, minor axes, theta angle, ellipticity, elongation, moments, fwhm), extraction flags and crude star-galaxy separation.

    -(n or f)d-dose
    Dosage map (total counts in detector coordinates)

    FITS 2-D image. Dimensions are x, y in detector coordinates. Units are in counts.

    Target level - merged catalog

    These files contain binary FITS tables which contain data that will be stored directly in database. Database metadata and table entries may also be extracted from FITS headers.

    -xd-mcat
    Merged source catalog (both bands)

    Primary image null. Binary FITS table (extension 1). Fields are: ObjectID, band merger outputs (combined ra, dec, match probability, separation, ambiguity, match flags) and combined catalogs from each band. As with the -cat file, fields for each band conform to standard (or modified) Sextractor outputs: Isophotal (standard and corrected), aperture, Kron, ``Best'' magnitudes and errors, Background, detection threshold, centroid position (image, world coordinates), peak position, shape parameters (semi-major, minor axes, theta angle, ellipticity, elongation, moments, fwhm), extraction flags and crude star-galaxy separation.

    Target level: grism-specific files

    • pri             (Not available in the ERO.)

      This file contains the image strips for individual exposures for each spectral-extracted source. The photon data, response, and masking images are recorded for each source. Masking for the image strip is stored in the response image (neighbor masking) and the photon data image (other masking) as negative numbers. Spectral response variations with position (column) are stored in the response image in the final row. These images contain the total accumulated photon data for each source for a given single visit or exposure. Each source has its own FITS header unit. The order of the image strips is identical to the order of sources in the extracted source catalog given in the (root)-gsax.fits file.

      First Header Unit: Grism angle used for these extractions, total exposure time, total number of spectrally extracted sources (ECOUNT), field center, and other extraction parameters.

      Second through ``N" Header Unit: Accumulated image strips for photon data and response for any of N-1 sources in the field of view. The header portion defines the image dimensions (PRI_NC,PRI_NR). The scale is given in arcseconds per pixel in both spatial and dispersion directions. The header key words also include the blue limit offset in arcseconds (ARCSEC1) relative to the object center (undeviated) position, which is used to derive the wavelength scale. The image strips typically cover multiple grism orders, e.g. 1st, 2nd, and 0th. The image data is stored as 2-byte integers, which can be scaled to true data values using the header key words DATZERO, DATSCALE, RSPZERO, and RSPSCALE.

    • prc

      This file contains the stacked or summed image strips for multiple exposures for each spectral-extracted source. The format is the same as for the pri files, except that masking has been applied before summing. That is, all masked pixels in the individual (pri) image strips were set to zero before summing.

      First Header Unit: Contains the filenames of all *-pri.fits which have been stacked, as well as their respective grism angles.

      Second through ``N" Header Unit: Accumulated image strips for photon data and response for any of N-1 sources in the field of view.

    • prm

      This file contains the medianed image strips for multiple exposures for each spectral-extracted source. The format is the same as for the pri files, except that masked pixels have been ignored by the median value calculation.

      First Header Unit: Contains the filenames of all *-pri.fits which have been medianed, as well as their respective grism angles.

      Second through ``N" Header Unit: Accumulated image strips for photon data and response for any of N-1 sources in the field of view.

    • gsax

      General information (binary FITS table). This file contains parameter information, as well as a source list table including extraction parameters for each source. This source list catalog is created on the first exposure and read in and used for each subsequent exposure. This file also contains a table of data for each accumulated exposure.

      First Header Unit: Header key words contain various extraction parameters for ``galaxspac" (spectral accumulation program). These include the flux cutoff (i.e., the brightest ``N" sources) used on the direct image source catalog, the field RA,Dec origin, the masking parameters, band number, the total number of exposures (or orbits) and exposure time accumulated.

      Second Header Unit: Extracted source list. This contains a source catalog of all the sources which have been extracted. This includes the RA,Dec positions, global IDs, direct image FUV and NUV flux rates, direct image FWHMs, and extraction parameters (object and background widths and length of the spectrum) for each source.

      Third Header Unit: Accumulated exposures (orbits) list. This contains a catalog of all the photon data sets added into the image strips. This includes all the *-pri.fits filenames, grism angles, field offsets used (x, y, twist (rotation) relative to the direct image source positions), exposure times, and reduction dates.

    • xsp

      An xsp file, labeled *-fg-xsp and *-ng-xsp, is extracted for each wavelength band. Each file represents two sequential spectral orders as recorded along the spectral axis on the detector, so units are photons per second per arcsecond. Each source has its own FITS header unit, in the same order as the data occur in the -pri.fits file.

      First Header Unit: Contains the header key word for the total number of sources extracted (ECOUNT).

      Second through ``N" Header Unit: Spectra for each source and its computed 1 sigma error. Spectra are given in photons/second/pixel vs. offset arcsecond (position relative to object center or undeviated object position).

    • gsp

      There are three gsp files for each spectrum: *-fg-gsp, *-ng-gsp, and *-xg-gsp. The first two are grism-order merged ( =1 and 2 for NUV, and 2 and 3 for FUV), flux calibrated, and rebinned onto a linear wavelength scale. The third is the conjoined spectrum of the first two. The units for these are photons/second/cm/Angstrom. The fg-gsp and ng-gsp files are for many purposes rendered obsolete by the xg-gsp file. Since the dispersion is linear, the zero point and dispersion of the wavelength scale are given as table entries. Two of the vector entries (flux and error) are for a simple (summation) extraction, and two vectors are for the optimal extraction.

      Each file consists of a short primary header (giving the number of spectra in the file) and a single extension. In this extension each row gives spectral information (default fluxes, associated errors, optimally extracted fluxes, associated errors) for an astronomical object. These rows are listed in order of their appearance in the extracted source catalog in the (root)-gsax.fits file. However, ``masked" sources appearing in the gsax file are not extracted as spectra and thus not included in the gsp file listing.

    For further information on the pipeline data description, goto CalTech's Pipeline and advanced data description
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    Q: What are the data column names?
    A: GALEX data column names are taken from the keywords in the GALEX extension or primary headers of the fits files, though sometimes we have tweaked the names of the columns to make their meanings a little more apparent. Descriptions of these names can be found in the Tables section of the "DB Info" item on the left gutter. If the "description" is also a link, clicking on it will take the user to a more extended definition.
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    Q: What is the meaning of the artifact flags in the images?
    A: The pixels in the artifact flag images are bitwise-encoded unsigned integers, each of which is the bitwise logical 'or' of some or none of a set of 9 possible flags. See Table 2 and the image artifact gallery at CalTech's Pipeline and advanced data description.The flags are applied to regions of the image derived entirely from geometry based on knowledge of the detector's orientation and (for some flags) an a priori estimate of the NUV/FUV brightness of known stars. Source brightness criteria for flagging are generous, meaning that false-positives, especially for 'edge' flags, are common. False-negatives are much rarer, but definitely occur with some regularity. As an example, if pixel N for an image is overlayed by an edge reflection region and is in the rim region (i.e. near the detector edge), its value is


    Flag(N) = 2(1 - 1) = 1 (edge condition)

    or 2(6 - 1) = 32 (rim condition)

    1 + 32 = 33 (bit coded value for both conditions)

    The flag should be treated as an unsigned value.


    In practice, the DICHROIC REFLECTION and WINDOW REFLECTION flags are the only flags that one should always select against as these could lead to seriously corrupt fluxes.

    Table 2
     
     Flag name   |   Bit   | Flag_N value |   Meaning                                 | Notes
                 | (LSB=1) |  (base 10)   |                                           |
     ------------|---------|----------------------------------------------------------|--------
     Edge        |    1    |   1          | Detector bevel edge reflection (NUV only) | 1,2
     ------------|---------|----------------------------------------------------------|--------
     Window      |    2    |   2          | Detector window reflection (NUV only)     | 1,2
     ------------|---------|----------------------------------------------------------|--------
     Dichroic    |    3    |   4          | Dichroic reflection                       | 1,2
     ------------|---------|----------------------------------------------------------|--------
     Varpix      |    4    |   8          | Varible pixel based on time slices.       |
     ------------|---------|----------------------------------------------------------|--------
     Brtedge     |    5    |  16          | Bright star near edge of field (NUV only).|
                 |         |              | Just like #1 but with a higher count rate | 
                 |         |              | requirement (about 10 cps) on the         | 
                 |         |              | generating star. Cuts down on false       | 
                 |         |              | positives at the cost of a higher false   | 
                 |         |              | negative rate.                            | 
     ------------|---------|----------------------------------------------------------|--------
     Rim         |    6    |  32          | Detector rim (annulus) proximity          | 3
                 |         |              | (>0.6 deg from FOV center).               | 
     ------------|---------|----------------------------------------------------------|--------
     Dimask      |    7    |  64          | Dichroic reflection artifact mask flag.   |
                 |         |              | Used only when a coadd has enough visits  | 
                 |         |              | at enough position angles that masking    | 
                 |         |              | the dichroic reflection doesn't decrease  | 
                 |         |              | flux by more than 1/3.                    | 
     ------------|---------|----------------------------------------------------------|--------
     Varmask     |    8    | 128          | Masked pixel determined by varpix.        |
     ------------|---------|----------------------------------------------------------|--------
     Hotmask     |    9    | 256          | Masked detector hotspots.                 |
    
    Notes:

    1) Flagged based partly on known star brightness estimate.

    2) NUV-only. In principal these artifact can (and do) appear for FUV images too. However, they are much rarer and have not yet been introduced into the processing pipeline.

    3) Not strictly an artifact, but a warning that the detector rim (edge) is near.



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